If you're in the temperature range of say, 10W-30, which is typically above 0 def F, then both oils should have approximately the same viscosity (in real life Castrol 10W-30 and Mobil 10W-30 synthetics are different, Mobil-1 is thinner)
Then 10W will have LESS viscosity modifiers than 5W, and when these modifiers gradually break down you won't end up with a thinner oil with insufficient oil film strength at operating temp.
Too much modifiers isn't a good thing, an old example is the good ol'
10W-40. Becuase of viscosity modifiers there is less oil in the 10W-40 than 10W-30 to help lubricate your engine!!!
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You know, that's the exact same logic I used when I first switched
from 5W-30 to 10W-30. Now because of information I learned about cold
> start wear, I'm leaning back to 5W-30. (Better fuel economy, too, in
> this age of $4/gallon gas.) It's all supposed to be the same
> viscosity when warm anyway, right? So cold starts should dictate
> which oil to use...???
>
> Michael